Some embodiments relate to a multiplexed fiber sensor, in particular a sensor configured to receive a signal from a hydrophone array and comprising an interferometer and a detector.
For approximately the last 40 years hydrophone arrays based on fiber optic and interferometric technologies have been under consideration. Compared with traditional piezoelectric hydrophones, fiber optic versions offer a number of advantages, including low power consumption, low cost, low weight and improved reliability, as well as low flow noise and insensitivity to EM (electromagnetic) interference. Single-mode fiber laser designs, such as distributed Bragg reflectors (DBR) and distributed feedback (DFB) arrangements have shown promise in detecting environmental perturbations, such as movement underwater. This makes them eminently suitable for use in hydrophone arrays, since such fiber lasers may be configured to show sensitivity to temperature and strain.
A DBR laser is typically formed by placing reflectors at either end of a length of a rare-earth doped fiber, for example, two Bragg gratings with identical reflection wavelengths coupled to an erbium-doped fiber. This sets up a simple etalon structure that with suitable excitation using another light source—“pumping”, typically at 980 nm or 1480 nm, causes a Fabry-Perot cavity to lase at a very specific wavelength typically between 1530 nm and 1560 nm, determined by Bragg grating centre wavelength, the length of the cavity and the emission bandwidth of the dopant (erbium). A DFB laser is essentially a simple version of the DBR structure, again forming a simple etalon. Two Bragg gratings are formed within the doped fiber and separated by a short length, which is less than one Bragg wavelength such that a phase step is produced within the length of the grating. Pumping the fiber at, for example, 980 nm again causes the Fabry-Perot cavity to lase at a specific wavelength. Both the DBR and the DFB laser act in the same manner when placed within an acoustic field, since the fiber itself becomes dynamically strained by the acoustic field, causing the Fabry-Perot cavity to change dimension, thus causing a change in wavelength. This change in wavelength can then be sensed using various methods and translated into information regarding the incident acoustic field.
One possibility for determining the wavelength shift and therefore the incident acoustic field effects is to employ an interferometer. In simple terms, an interferometer determines information about waves by superposing them, typically after splitting the incident wave into two and utilising two arms to produce a variation in one of the waves. For example, both the Mach-Zender interferometer and the Michelson interferometer employ amplitude splitting, where a partial reflector is used to divide the amplitude of the incident wave into separate beams which are separated and recombined. This makes such sensors ideal for looking at the wavelength, frequency and phase shifts induced in beams of laser light when a laser cavity undergoes a length change under the influence of an acoustic field.
An example of this is a simple Mach-Zender interferometer (MZI) set up utilised in a wound coil type fiber hydrophone, such as the LWWAA (Light Weight Wide Aperture Array) system sold by Northrup Grumman (2980 Fairview Park Drive, Falls Church, Va. 22042, USA), which employs two wound fiber coils, one acting as a reference mandrel and one as a sensing mandrel. This balances the interferometer and determines the change in wavelength on exposure to an acoustic field. In this case, the interferometer is used as the sensor rather than the readout device. The sensing mandrel is in the sensing zone, that is the region where the acoustic field is incident on the fiber laser array and is sensed. One issue with using the MZI to read such changes directly is that a large path imbalance is required between the arms of the interferometer, which may lead to issues of size constraints in use.
An alternative Mach-Zender system is described in Hill et al., SPIE vol. 3860, pp 55 to 66, 1999, where the interferometer is used to read out the changes in the fiber laser array. The pressure acting on the fiber causes a wavelength change that is proportional to the fractional apparent length change of the laser cavity, which is typically 40-60 mm long length of fiber with gratings at either end. The wavelength shift is converted to a measurable phase shift in a Mach-Zender interferometer with a large path imbalance between the two arms, such that the large path imbalance is in the read-out zone and not the sensing zone. Whilst this offers some advantages over placing the wound coil directly in the sensing zone, introducing such a large path imbalance can lead to issues with signal noise.
A further alternative is to use a Michelson interferometer as the sensor for the hydrophone array, rather than a Mach-Zender interferometer. Such a sensor in conjunction with a DFB laser array is described in Foster et al., SPIE vol. 5855, pp 627-630, 2005. In this case an array of pumped DFB lasers sends a signal to a Michelson interferometer where a splitter/combiner sends signals received from the laser array down one arm containing a delay loop and one arm containing an acoustic-optic modulator (AOM) to generate a phase modulation for a carrier signal. Both arms result in a single output signal that is then sent to a DWDM (dense wavelength division multiplexer) and undergoes demodulation processing to recover the change in wavelength of the fiber lasers due to the incident acoustic field. Faraday mirrors are employed at the end of each interferometer arm, which cause the reflected beams to reverse any polarisation effects on their return to the splitter/combiner used to send the input beams down each arm of the interferometer. This gives optimal signal mixing in the processing stage.